Means (negative pressure method) for judging a leak in accordance with pressure changes occurring when an evaporation purge system is placed under negative pressure is well known as a conventional technology for detecting a leak from the evaporation purge system for a fuel tank of an automobile or other vehicle (e.g., Japanese Patent JP-B 2741698 and Japanese Patent JP-A 189825/1995).
However, the use of the negative pressure method has not provided an adequate diagnosis frequency in the market. One reason is that the negative pressure is insufficient during a high-load operation. To solve this problem, a method for installing a pump in a purge line between a canister and intake pipe and operating the pump to generate a negative pressure for diagnosis purposes is disclosed (refer, for instance, to Japanese Patent JP-A 138910/2002 and Japanese Patent JP-A 349364/2002).
To increase the diagnosis frequency in the market, however, it is important to establish measures for preventing a diagnosis error due to evaporation, and more particularly to establish measures for preventing a diagnosis error due to fuel level undulation (slosh). The above-mentioned technologies do not provide any solution to establish such measures.
A conventional evaporation purge system for a fuel tank will now be described with reference to FIG. 23.
A fuel tank stores gasoline or other liquid hydrocarbon fuel. The fuel tank 500 is equipped with a liquid level sensor 501. The liquid level sensor 501 converts a fuel level into an electrical signal, which enters a control unit 503. The fuel tank 500 is equipped with a pressure sensor 502, which measures the internal pressure of the fuel tank. The pressure sensor 502 converts the pressure difference between the vapor section of the fuel tank 500 and the outside of the fuel tank (atmospheric pressure) into an electrical signal and feeds the electrical signal to the control unit 503.
An evaporation purge system includes a canister 504. The canister 504 contains and adsorbent that adsorbs and retains fuel vapor. A communicating tube 510 is connected between the canister 504 and the vapor section of the fuel tank 500. Vapor that arises when the fuel in the fuel tank 500 evaporates is adsorbed and retained by the canister 504. The canister 504 prevents the vapor from being discharged into the atmosphere.
The canister 504 is equipped with a drain control valve 505. The drain control valve 505 opens and closes in accordance with an electrical signal from the control unit 503.
Therefore, when the drain control valve 505 opens while the vehicle is driven, the fuel vapor generated within the fuel tank 500 is introduced into the canister 504 and adsorbed by the canister 504.
The canister 504 communicates with an intake pipe 509 of an internal combustion engine 508 via a communicating tube 507. A purge control valve 506 is installed in the middle of the communicating tube 507. The purge control valve 506 is driven by an electrical signal from the control unit 503 so that the opening of the purge control valve 506 can be controlled.
The internal combustion engine 508 generates a negative pressure in the intake pipe 509 within a low- to medium-load operating range. Therefore, when the opening of the purge control valve 506 is adjusted as specified with the drain control valve 505 opened in such a state, outside air is introduced into the canister 504. The adsorbed vapor is then desorbed and introduced into the intake pipe 509. The introduced outside air and vapor are burned together with normal intake air and supplied fuel in the internal combustion engine 508.
A leak detection sequence based on a conventional negative pressure method will now be described with reference to FIG. 24. FIG. 24 is a typical timing diagram. This diagram illustrates operations that the drain control valve 505 and purge control valve 506 perform for evaporation leak diagnosis as well as a tank pressure that is detected by the pressure sensor 502.
Between time A and time B, the drain control valve 505 closes and the purge control valve 506 opens. The negative pressure generated in the intake pipe 509 of the internal combustion engine 508 then reduces the pressure within the fuel tank 500. When the tank pressure decreases to a predetermined level (at time B), the purge control valve 506 closes to hermetically seal the evaporation purge system. If the gradient of tank pressure increase is great before time C is reached, it is judged that there is a leak. Further, the diameter of a leak hole is determined according to the magnitude of the gradient.
However, if a slosh occurs during the use of the conventional negative pressure method, a leak may be erroneously detected due to the influence of evaporation pressure even when there is no leak. Therefore, when the conventional negative pressure method is used, it is necessary to detect a slosh and disable or mask the diagnostic function. That is why it has been difficult to provide an adequate diagnosis frequency in the market.
The present invention has been made in view of the above circumstances. It is an object of the present invention to provide an evaporation leak diagnostic device that is capable of making diagnoses without being affected by a slosh, and increase the diagnosis accuracy and frequency in the market. It is another object of the present invention to judge the properties of the fuel in the fuel tank by making effective use of information generated by the evaporation leak diagnostic device, and provide increased stability and improved exhaust at the time of engine startup.